1. Field of the Invention
The present invention relates to coating die which is used for drawing an optical fiber, and to an optical fiber drawing method therefor.
2. Description of Related Art
It is common for a plurality of resin coating to be applied to an optical fiber from a viewpoint of maintaining mechanical strength and transmission characteristics. An ultraviolet curable resin having relatively lower Young's modulus are applied to an optical fiber as a first coating layer and an ultraviolet curable resin having relatively higher Youngs modulus and a second coating layer.
An optical fiber drawing apparatus for applying an ultraviolet curable resin is explained with reference to FIG. 2 as follows.
In FIG. 2, a reference numeral 1 indicates an optical fiber. The optical fiber 1 is formed by performing heat-melting processing so as to extend a raw material 2 of an optical fiber in a drawing furnace 3. A liquid first ultraviolet curable resin is applied to an outside of the optical fiber 1 by passing the optical fiber 1 through a first resin coating apparatus 4. Furthermore, by passing the optical fiber 1 through a first hardening apparatus 5 (UV lamp) for irradiation with ultraviolet light, the resin is hardened; thus, a first coating resin is formed on the optical fiber 1.
Consequently, a second coating resin layer such as an ultraviolet curable resin is formed on the first coating resin layer by passing the resin-coated optical fiber through a second resin coating apparatus 6 and a second hardening apparatus 7. A resin-coated optical fiber 8 is wound by a winding apparatus 9. The resin coating process is performed vertically.
Recently, it is required that the productivity of the optical fiber be improved in accordance with an increase in demand therefor. In particular, high speed ultraviolet curable resin coating processing is required to be realized. However, if high speed coating processing is realized, there is a problem in that ultraviolet curable resins cannot be applied stably and in uniform thickness.
For a solution to the above-mentioned problem, a coating process which is described in Japanese Examined Patent Application, Second Publication No. Hei 7-5336 is known. According to this coating process, defects in the coating resin is prevented from occurring in coating die by controlling the application temperature of the liquid ultraviolet curable resin and by controlling the viscosity of a region of the liquid ultraviolet curable resin, in which shear rate is slower than a critical shear rate, in the applying temperature with in a predetermined range. In this coating process, it is disclosed that the viscosity is in a range of 500 cps to 3000 cps in an application temperature of 60° C. to 100° C. in a region in which shear rate in the applying temperature is slower than a critical shear rate.
However, as disclosed in the above-mentioned prior art document, shear rate of an ordinary ultraviolet curable resin is in a range of 104 to 105 sec−1. If the drawing process is performed while the shear rate must be slower than the critical shear rate, the line speed cannot be high. If a high speed drawing such as a line speed of 10 m/sec is performed, interfacial shear rate becomes quite high; thus, it easily becomes higher than its critical shear rate.
On the other hand, designing aspect of coating die becomes important along with an increasing drawing speed of the optical fiber. Japanese Examined Patent Application, Second Publication No. Hei 7-91092 is an example of such design for coating die. A coating die which is disclosed in this prior art document is explained with reference to FIGS. 3, 4, and 5 as follows.
In FIG. 3, reference numeral 1 is an optical fiber. A resin 12 is applied to the optical fiber 1 while the optical fiber 1 is passing through a guiding die 10 and a die 11. Reference numeral 13 indicates a holder for supporting the guiding die 10 an the die 11. Reference numeral 14 indicates a guiding die hole. Reference numeral 15 indicates a bottom face of the guiding die. Reference numeral 16 indicates a tapered section of the die 11. Reference numeral 17 indicates an exit hole of the die 11. Reference numeral 18 indicates a top face of the die 11. A reference numeral 19 indicates a meniscus.
In FIGS. 4A and 5A, the guiding die 10, a die 11, and shapes thereof are shown. In the above-mentioned prior art document, the relationship between B and H in FIG. 4, the relationship between G and C, the relationship between G and D, and the range of angle α are disclosed.
In the above-mentioned prior art document, a case in which a tapered section of the die 11 has two steps by modifying a structure shown in FIG. 4B is mentioned. However, the detail of such structure is not disclosed. Also, a case in which a tapered section of the dies 11 has a curve by modifying a structure shown in FIG. 5B is mentioned. However, the detail of such structure is not disclosed.
Also, the required shape of the die 11 is different between a case in which a resin is applied to an optical fiber and a case in which a resin is further applied to the resin-coated optical fiber. Furthermore, in actual drawing operation, while the drawing speed increases from a low speed such as a starting speed (for example, 0.5 m/sec) to a high speed in which a product is drawn on an operational basis (for example, 30 m/sec), temperature of an optical fiber which is coated by a resin is not fixed. Therefore, coating conditions becomes unstable to a certain degree. In order to obtain a high quality optical fiber, coating operation must be performed stably so as to avoid slipping phenomena of resin in each drawing speed range.